Optical networks are commonly installed to provide a TPON (Telephony on Passive Optical Network) service. In a typical TPON system the transmission of data is synchronized by a masterclock in the exchange. At the customer end the clock is recovered using a conventional phase lock loop (PLL). When data is transmitted back from the subscriber station to the central exchange it is possible to time the transmission to an accuracy of one tenth of a clock cycle: at a typical TPON bit rate of 20 Mbit/s this requires an accuracy of .+-. 5 ns. The required timing accuracy is readily achievable using conventional circuitry of modest cost. Since the system clock is accurately recovered and used by the subscriber stations for the transmission of data back to the exchange, the exchange can simply use its local clock in handling the incoming data and no further clock recovery is needed. A single clock is used throughout the system making it possible to use the TPON in bit interleave mode which reduces the demands made on the subscriber station laser and brings benefits in terms of cost and reliability.
One example of such a system in which subscriber stations recover the masterclock frequency from received data, and transmit data cells back to the exchange at that masterclock frequency is disclosed in the present Applicant's published earlier application no. WO-A-88/05233. A further feature of this system is that control signals may be sent from the exchange to a subscriber station to advance or retard the timing of transmission from the subscriber station.
In practice rather than having a network dedicated to telephony it is desirable to be able to use the system at the same time for other forms of traffic such as high bit-rate data transmission at a wavelength separated from that used for telephony. However at the data-rates typically used for such traffic, which may, for example, be around 150 Mbit/s, it proves difficult to recover the clock with sufficient speed. It is possible for the subscriber stations to retrieve the clock accurately using a conventional PLL and this is sufficient for the reception of data from the exchange. However the recovered clock has an arbitrary phase difference with respect to the masterclock dependent on the transmission distance between the exchange and the subscriber station. The realignment of the phase to an accuracy of the order of one tenth of a clock cycle or .+-. 0.6 ns in the time available could only be achieved by using circuitry of a cost and complexity wholly unsuitable for commercial applications. It can, for example, be shown that to recover the clock with such an accuracy within the few bits of a clock lead-in using a conventional PLL would require a voltage control oscillator capable of covering the range from 74 to 226 MHz, which is beyond the capabilities of most such devices. Moreover the transmission of data with this degree of timing accuracy would also require the use of frequent timing updates to correct for variations in fibre delay characteristics caused by temperature fluctuations.
By contrast with the systems described above, U.S. Pat. No. 4,012,598 discloses a receiver for use in a system with independent clocks in the receiver and in transmitting station. Since the frequency of the receiver clock is in general imperfectly matched to that of the transmitter clock there is a continual phase drift in the received data. In order to correct for this drift a delay line is provided with different output taps. For each pulse of the received data the phases of the different current outputs and preceding outputs are compared with the local clock and the output with optimal pulse durational phase chosen. Since the phase drift is continual a fresh determination has to be made for every pulse.